What kind of weights do people have on their bikes on solo camping tours?
Last summer, for a weekend of cycling and camping on the atrocious C&O trail here in Maryland, I put a total of probably 250lbs or so on my 1995 Jamis Aragon (an old relatively cheap hybrid) and pulled my daughter on a Trail-a-Bike, so dragging another 80lbs. Although the C&O trail is my idea of cycling hell (being a dirt and loose stone track, liberally sprinkled with deep water-filled potholes), the bike handled the constant shaking, skidding, hammering and juddering without a problem - not even a puncture. Certainly no structural issues with the frame.
Balance shouldn't be much of a problem as long as you have the weight evenly distributed laterally. You tend to get used to the extra weight pretty quickly (on the first day's ride). Opinion is divided on the issue of whether the weight is better low down - some say it is, others say it doesn't really matter. I've even heard some say that higher is better. I think it's better lower, but then again, I spent a year and a half touring with at least 50lbs of gear stashed mostly on top of my rear rack, and the high centre of balance didn't bother me. I must admit, I've never spent much time or effort worrying about the centre of balance. It is where it is, and I deal with it.
And look at it this way - whatever your gear weighs, you probably weigh something like twice as much, and you're very far above the bike's centre of balance, and you're not even properly tied down.
And when Surly say 300lbs max, standard engineering practice (at least when I was working as a structural engineer/draughtsman) was to build in three times the estimated load capacity. So when Surly says 300lbs, the bike might take 900 before it actually breaks. But that doesn't mean Surly will pay a penny or fix it if you break it while carrying 301lbs on it.
I'm pretty sure this bike was probably designed to carry 200lbs max - and I suppose one can only hope that that's all just polystyrene:
But I'm pretty sure this ain't: